Construction of D’Arsonval Galvanometer
D’Arsonval Galvanometer consists of two main parts
- A permanent magnet
- A light coil
Here we use a very lightweight rectangular or circular aluminum frame. Then, on that aluminum frame, we wrap fine insulated wire. It is the coil of the instrument. We use many turns in the coil. We suspend this coil axially with the help of a phosphor bronze filament. Also, this phosphor bronze filament acts as one terminal (lead) of the coil.
There is a very flexible spiral filament at the bottom of the coil. This bottom spiral filament acts as the second terminal of the coil. In addition to providing terminal of the coil, the elasticity of the filament provides controlling torque during rotation of the coil.
The space between the pole faces of the permanent magnet is usually cylindrical in shape. This is to facilitate the rotation of the coil. Also, it is to provide a smooth path to magnetic flux.
We may use a cylindrical stationary iron core in between the pole faces. This cylindrical iron core should be coaxial with the moving coil.
Advantages of Providing Cylindrical space between the pole faces
- It provides the uniform air gap between the coil sides and pole faces. That means the gap between the coil sides and pole faces remains the same at every angular position of the coil.
- This arrangement reduces the gap between coil sides and pole faces at every angular position of the coil. Therefore it increases the magnetic linkage with the coil. Consequently, the sensitivity of D Arsonval galvanometer increases.
- The pole face emits flux radially. Hence at every angular position of the coil, its conductors cut the flux perpendicularly. Therefore, the deflection torque of the coil is directly proportional to the current through it. Therefore we can obtain a uniform scale of the instrument.
Working of D’Arsonval Galvanometer
Whenever we place a current carrying conductor in a magnetic field, a force acts on the conductor. The working principle of D Arsonval Galvanometer depends on this basic principle.
When we connect the galvanometer with a live circuit, current starts flowing through the coil. Depending on the current, the strength of the permanent magnet and the dimension of the coil, the coil experiences a deflecting torque. As a result, the coil deflects along with the pointer from its de-energized position. The coil rotates along its vertical axis.
During this rotation the suspension filaments also get twisted. Therefore, this twisted filament creates an opposite torque on the coil. We call this opposite torque as restraining torque. Obviously, the restraining torque depends on the angle of deflection of the coil. Therefore after a certain angle of rotation of the coil, the restraining torque becomes exactly equal and opposite of deflecting torque. This is the balance condition of the moving system of D Arsonval Galvanometer. Therefore the pointer attached to the moving system stops at the deflected position. And it remains there until the steady current flows through the coil.
Calibration of D’Arsonval Galvanometer
The deflection torque depends on the strength of a permanent magnet and the dimension of the coil. Also, it depends on the amount of current flowing through the conductors of the coil. Since the magnetic field strength and dimension of the coil are fixed; the deflecting torque is directly proportional to the current through the coil. So each angular position of the coil depends on the amount of current flowing through it. Therefore, we can easily calibrate this instrument with the corresponding current, at its every deflected position. There is a threshold head to adjust the coil (moving system) for zero position.
Scale of D’Arsonval Galvanometer
We can attach a small mirror to the moving system to indicate the angular position of the coil more promptly. Often, we can use a telescope and a scale in front of the D Arsonval galvanometer to read the scale reflected by the mirror. Again sometimes we use a lamp. The mirror reflects the light of the lamp on the scale to spot the exact reading of the galvanometer. In that case, we keep the scale approximately one meter away from D Arsonval galvanometer. Although sometimes we use the galvanometers with scale only 0.5 meters away. This is only because of the compactness of the instrument.
Damping Torque of D’Arsonval Galvanometer
Whenever there is a movement, eddy currents are induced in the aluminum frame of the coil. So eddy current damping is readily available in the instrument. Therefore, D Arsonval Galvanometer normally does not require any additional arrangement for damping. Also, we may connect one low adjustable resistance across galvanometer terminals. We can obtain critical damping by adjusting this resistance.
Torque equation of D’Arsonval Galvanometer
N is the number of turns of the coil,
B is the flux density between the permanent magnetic poles.
I is current to be measured that is flowing through the coil.
L is the length of the coil side.
b is the width of the coil.
θ is the angle of deflection of the coil during measurement.
K is the spring constant of spiral suspension filament.
Now, the force acting on each conductor of the coil is BLI.
Since the coil has N turns, therefore, the total force will be NBIL. Due to the presence of stationary iron core in the space between the poles, the flux lines become radial in that space. Hence, at every angular position of the coil, the force acting on the coil sides is tangential. Means the farce is always perpendicular to the plane of the coil.
Here N, B, L, b are constant
We call G as displacement constant of the galvanometer.
Now for deflection angle θ, the restraining torque or controlling torque
At balanced condition
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